Field of the Invention
The invention relates to an analog/digital converter operating on the principle of successive approximation and having a capacitor network.
Analog/digital converters are known, for instance, from U.S. Pat. No. 4,399,426 and U.S. Pat. No. 4,451,821. Analog/digital converters with a capacitor network are often used whenever neither the analog input nor the reference voltage sources are to be loaded by a static current. Moreover, the capacitor network already furnishes a storage capacity, so that a sample-and-hold circuit preceding the analog/digital converter can be dispensed with.
During a transfer phase, the analog input voltage to be measured is applied to the capacitors. During that time, a voltage at inputs of an offset-compensated comparator is stored in memory. In the conversion phase, a charge at the capacitor network is redistributed, by alternating application of two reference voltages, long enough to ensure that the voltage at the inputs of the comparator is equal, except for a quantization error, to the voltage during the transfer phase.
The position of the reference switch is then equivalent to the digitized value of the analog input voltage.
However, one disadvantage of that converter is that the input voltage region is coupled directly to the reference voltages. At a lower reference voltage of a volts and an upper reference voltage of b volts, an input voltage of a volts, for instance, is converted into a digitally encoded value 0, and the voltage of b volts is converted into a maximum codeable digital value, which is dictated by the number of digits in the digital value.
If, for instance, at the reference voltages of a volts and b volts one wishes to measure a voltage that can be located between a volts and 2.times.b volts, then the input voltage to be measured must first be put in the appropriate voltage range by a voltage divider. However, with an ohmic voltage divider the converter loses its property of not ohmically burdening the input voltages to be measured.